1. Field of the Invention
The present invention is related to nanofibers, methods and devices for electrospinning, methods and devices for depositing the nanofibers, and filters and other articles formed from the deposited nanofibers.
2. Description of the Related Art
The filtration industry has traditionally manufactured particulate air filters using conventional medium such as glass, cotton or polymer fibers made provided as rolled goods. The fibrous media may be made by non-woven processes such as wet laid paper, melt blown-spinning or woven yarn. The material is then transported to equipment where the media is cut, pleated, supported, glued into filter frames, and tested for leaks. Various measures of the properties of the rolled goods include appropriate weight per unit area, porosity, etc.
The porous filter media may be pleated or bonded into bags to increase the area of the media within individual filter units to reduce pressure drop. Often screens and other supports are added to prevent collapse of the media from the force of air flowing through the filter unit as dust is collected. Depending on the intended use of the filter, the filter may be tested with an appropriate challenge aerosol at a rated or standard airflow rate for pressure drop and particle collection efficiency. (e.g., ASHRAE 52.2, MIL-STD-282, IEST RP-CC 007.1, NIOSH APRS-STP-0051-00, and NIOSH APRS-0057-00 may be used to test the filters)
Theoretically, a reduction of the diameter of the fibers in a filter has the potential of causing an improvement of the filter system performance. For high efficiency filtration, fiberglass wet-laid papers are widely used having fiber diameters in the 200 nm to 5000 nm size range with the fiber sizes intentionally blended for both durability and filtration performance.
One technique for producing a smaller fiber diameter, and hence a potential for generating improved filtration media, is electrospinning of polymers to make submicron and nanofibers. Electrospinning as currently practiced uses a constant voltage to drive the spinning process defined herein as static field electrospinning.
However, electrospun nanofibers smaller than 500 nm are typically fragile, difficult to produce, and difficult to handle. One conventional approach has been to deposit nanofibers onto a conventional porous filter media to make a layered nanofiber filter media. The following patents describe conventional ways to fabricate nanofiber containing filters for various applications: U.S. Pat. Nos. 7,008,465; 6,994,742; 6,974,490; 6,955,775; 6,924,028; 6,875,256; 6,875,249; 6,800,117; 6,746,517; 6,743,273; 6,740,142; 6,716,274; and 6,673,136, and U.S. patent application Ser. Nos. 10/757,924 and 10/676,185; the entire contents of each of these patents are incorporated in entirety herein by reference.
Conventional layered nanofiber filters made from nanofibers deposited on conventional porous filter media have inherent limitations. The support media of these filters is usually pliable enough to allow pleating or manipulation during the assembly step. Such a pliable substrate media may flex or stretch from the air pressure drop force and may break or debond the nanofibers. The support layer of conventional media may contribute substantially to the pressure drop of the whole structure.
An ideal particulate filter is the one that would give the highest particle collection efficiency (lowest particle penetration) with the least pressure drop. One criterion for comparing filters of different thickness is the filter quality factor or figure of merit (FoM). The greater the value of FoM, the better the filter will perform (Hinds, 1982). One expression for this parameter is given by:FoM=−Log(Pt)/ΔP  (1)where: Pt is the fractional penetration of a specific aerosol particle diameter (efficiency=(1−Pt)), and ΔP is the pressure drop corresponding to a specific face velocity of the filter (volumetric air flow divided by filter cross sectional area). As used herein, figure of merit given by −Log(Pt)/ΔP, where Pt is the fractional penetration of a specific aerosol particle diameter and ΔP is a pressure drop across the filtration medium corresponding to a face velocity of 5.3 cm/s and particle size of 0.3 microns.
Typically, the FoM of a high efficiency particulate air (HEPA) glass fiber media is 12 kPa−1 measured at a face velocity of 5.33 cm/s and 0.3 μm particle diameter. These are the standard conditions for HEPA media tests (i.e., IEST-RP-CC021.1).
The FoM of the layered nanofiber conventional porous filter media described above is limited by the relatively large fiber diameters of the coarse substrate which produce a relatively low FoM. The FoM of the layered nanofiber conventional porous filter media composite depends on the relative quantities of layers of nanofibers and conventional media and their respective FoM. In other words, while the individual layers of nanofibers may have a higher FoM than the conventional porous filter media substrate, the composite FoM is closer to the value of the convention porous filter media substrate because of the relative quantities of materials used in the conventional approach. Therefore at the current state-of-the-art, conventional layered nanofiber filter media do not provide filters with significantly greater FoM than conventional fiberglass media.
References describing various background materials and filter testing procedures include:
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